In a variety of industries, a variety of attempts has been made worldwide to reduce environmental impacts and burdens. In particular, in the automobile industry, development for promoting the spread of not only fuel-efficient gasoline engine vehicles, but also eco-cars, such as hybrid vehicles or electric vehicles, as well as for further increasing the performance of such vehicles has been advanced day by day.
By the way, an exhaust gas exhaust system, which connects an engine in the vehicle and a muffler, may sometimes have mounted thereon an electrically heated catalytic converter for, in addition to purifying exhaust gas at room temperature, purifying exhaust gas when the temperature is low by activating a catalyst as quickly as possible through electrical heating. Such an electrically heated catalytic converter has a configuration in which, for example, a pair of electrodes are attached to a honeycomb catalyst, which is arranged in the exhaust gas exhaust system, and the pair of electrodes are connected with an external circuit having a power supply, so that the honeycomb catalyst is heated when electricity is conducted through the electrodes, and thus, the activity of the honeycomb catalyst is increased and exhaust gas that passes through the honeycomb catalyst is detoxified (i.e., electrically heated catalytic converter: EHC).
A typical configuration of the aforementioned electrically heated catalytic converter will be described. The electrically heated catalytic converter may include an outer tube (i.e., metal casing), a heat-generating honeycomb substrate, which has a catalyst coating layer arranged in the hollow space, an insulating mat (i.e., holding material) interposed between the outer tube and the substrate, a pair of electrodes that are attached to a region, which has no mat, of the surface of the substrate, for example, and an external circuit that connects the pair of electrodes.
More specifically, the electrically heated catalytic converter includes an electrode film, which is adapted to diffuse an equal amount of current as soon as possible across the entire substrate, at a portion where each electrode is provided, and an electrode terminal that is attached to the surface of the substrate via an opening provided in the electrode film. An external electrode is attached to the electrode terminal, and an external circuit is formed with a cable connecting to a power supply. It should be noted that a configuration is also known in which an opening is not provided in an electrode film, but an electrode terminal is attached to the surface of the electrode film.
As described above, the EHC includes a honeycomb substrate on which a catalyst is supported, an electrode film formed on the surface of the substrate, an electrode terminal formed on the surface of the electrode film, and an external circuit. Among them, the types of the substrate can be largely divided into a metal substrate and a ceramic substrate. Of the two, it has been found that a metal substrate, which has too low resistance, is difficult to be applied to hybrid vehicles or plug-in hybrid vehicles. Thus, application of an EHC that has a substrate made of ceramics, such as SiC or a composite material of SiC and Si, which can be applied even to environmentally-friendly vehicles, has become the mainstream.
Herein, each of the aforementioned electrode film, electrode terminal, and honeycomb substrate that are the constituent elements of the EHC is required to have the following operation and function.
First, the electrode film is required to have a collector function, and to this end, the electrode film desirably has lower volume resistivity than the substrate. In addition, the electrode film is also required to have a function of diffusing current, and thus is desirably able to promote conduction of an equal amount of electricity across the entire substrate, and, to this end, the electrode film is desirably able to diffuse and rectify an equal amount of current across the entire substrate as soon as possible. In addition, as the electrode film is attached to the surface of the substrate, the interface of the electrode film with the substrate desirably has strength against thermal stress that is greater than or equal to the strength of the substrate. To this end, it is desirable that not only should the connection strength be high, but also the thermal expansion coefficients of the substrate and the electrode film be close to each other so as to reduce the amount of heat deformation as soon as possible. Further, considering the resistance against thermal shock, the thermal conductivity of the electrode film is desirably greater than or equal to that of the substrate, and from the perspective of ensuring the environmental resistance and reliability, a change in the volume resistance of the electrode film under a high-temperature oxidation atmosphere is desirably low.
The aforementioned operation and function that are required of the electrode film are also true of the electrode terminal.
Meanwhile, with respect to the honeycomb substrate that is a heat-generating body, the resistance value is desirably controllable to a value that is optimum for the intended use as well as the supplied current and voltage. In addition, the substrate desirably has low temperature dependence of resistance at −30 to 1000° C. that is the temperature range in which a catalyst is used, and desirably has little change in the resistance value. Further, the substrate desirably has high oxidation resistance, high resistance to thermal shock, and the ability to be easily bonded to the electrode film or the electrode terminal.
So far, it has been found that when each of the electrode film, the electrode terminal, and the honeycomb substrate, which are the constituent elements of the EHC, is molded using the aforementioned ceramics, such as SiC or a composite material of SiC and Si, it would be quite difficult to meet the aforementioned requests for each of the constituent elements. Thus, it is desired in the art to produce an EHC that can entirely satisfy the variety of the required performance by developing the technology of the material for forming each constituent element.
Herein, Patent Literature 1 discloses a honeycomb structure in which a cell wall of a substrate with a honeycomb structure made of SiC contains silicon, silicide, or a mixture of silicon and silicide with low electrical resistivity.
Patent Literature 2 discloses noble-metal-based composite powder containing 0.5 to 30 parts by weight of ceramic powder with respect to 100 parts by weight of metal powder, which includes 35 to 90 parts by weight of silver, 5 to 30 parts by weight of palladium, and 5 to 50 parts by weight of metallic silicon, and also describes producing an electrode, a noble metal-ceramic composite, a heater, a diesel particulate filter, or the like using such noble-metal-based composite powder. Further, an embodiment of Patent Literature 2 describes formulating silicon carbide powder (with a purity of greater than or equal to 99% and a grain size of less than or equal to 5 μm) or molybdenum disilicide (with a purity of greater than or equal to 99% and a grain size of less than or equal to 5 μm) as ceramic powder with respect to 100 parts by weight of metal powder.
Patent Literature 3 discloses a honeycomb structure having a cell formation part, a honeycomb body with an outer skin part of a cylindrical shape that covers the cell formation part, and a pair of electrodes that are arranged opposite to each other in the radial direction of the honeycomb body on the outer circumference of the outer skin of the honeycomb body, in which each electrode has formed thereon an electrode terminal at the central part in the circumferential direction of the electrode, and the thickness of each electrode is gradually decreased from its central part toward the outside along the circumferential direction of the honeycomb body. Patent Literature 3 describes that ceramics containing SiC or SiC—Si, which is obtained by impregnating SiC with Si (metallurgical silicon), is used as the material of each electrode. In addition, Patent Literature 3 describes that the honeycomb body (i.e., substrate) is made of porous ceramics containing SiC as a main component, and also describes that the electrode terminal is formed of the same material as the electrode.
Patent Literature 4 discloses a honeycomb structure including a porous partition wall, an outer wall located on the outmost circumference, a cylindrical honeycomb structure portion made of a ceramic material containing SiC particles that serve as an aggregate and Si that serves as a binder for binding the SiC particles, a pair of electrode portions arranged on the side face of the honeycomb structure portion, an electrode terminal protruding portion arranged on the surface of each of the pair of electrode portions, and a metal terminal portion made of a metal material electrically connected to each electrode terminal protruding portion, in which each of the pair of electrode portions and the electrode terminal protruding portions is made of a conductive ceramic material containing SiC or Si as a main component, and each electrode terminal protruding portion and each metal terminal portion are electrically connected via a brazing material.
Patent Literature 5 discloses a silicon carbide heat-generating body including a heat-generating member that contains SiC and generates heat when electricity is conducted therethrough, a SiC—MoSi2 end portion member that contains a composite material of SiC and MoSi2 and is produced as a separate member from the heat-generating member and is then coupled to the heat-generating member for conducting electricity through the heat-generating member, and a SiC—Si end portion member that contains a composite material of SiC and Si and is bonded to a further end portion side of the SiC—MoSi2 end portion member, in which the content of MoSi2 in the SiC—MoSi2 end portion member is greater than or equal to 25 weight % and less than or equal to 35 weight %.
Herein, as a method for molding the SiC—MoSi2 end portion member using an extrusion press, a mixture of SiC and an organic binder is molded into a pipe-shaped object using an extrusion press, and is then dried and baked to obtain a recrystallized SiC sintered body, and further, pores of the obtained SiC sintered body are melt-impregnated with MoSi2.
In each of the techniques disclosed in Patent Literature 1 to 4 above, an electrode terminal, an electrode film, and the like are molded with ceramics such as SiC or SiC—Si. Thus, there still remains a problem that the aforementioned requests cannot be met.
In the technology disclosed in Patent Literature 5 above, the content of MoSi2 that forms the SiC—MoSi2 end portion member is greater than or equal to 25 weight % and less than or equal to 35 weight %. The grounds that the content of MoSi2 is defined as above are estimated to be due to the fact that when the SiC—MoSiz end portion member is literally located at the end portion, and the content of MoSi2 is set in a higher range than the upper limit of 35 weight %, the volume resistance of the end portion member would become too low, with the result that a heat-generating function as a heater will not be exerted.
However, the SiC—MoSi2 member whose MoSi2 content is defined in the range as low as less than or equal to 35 weight % is practically quite difficult to be applied as a constituent member of the EHC. This is because the constituent member of the EHC is required to have low volume resistivity and high oxidation resistance as described above.